As a conventional wireless communication system, there are multicarrier modulation systems represented by, for example, OFDM (orthogonal frequency division multiplexing) and DMT (discrete multitone), and these systems are used for wireless LAN, ADSL, and the like. In these wireless communication systems, carriers are arranged orthogonal to a plurality of frequencies and transmitted. As a characteristic thereof, for example, these systems include a guard interval or a cyclic prefix as a function for removing an influence of delay waves generated due to a propagation path between transmitters and receivers. In the receiver, the influence of delay waves in the guard interval is removed by performing FFT with respect to an OFDM symbol, from which the guard interval is removed, to demodulate data accurately.
On the other hand, in the OFDM, when OFDM subcarriers are not completely orthogonalized due to a frequency offset, timing offset, or nonlinear distortion, the characteristic deteriorates.
Further, in the OFDM, in a state where delay waves with an insufficient guard interval arrive, intersymbol interference and inter-carrier interference occur, thereby largely deteriorating the characteristic. This problem can be solved by adding a guard interval longer than a presumed delay time in the transmitter. However, in this case, overhead of the guard interval increases, and transmission efficiency decreases.
In an orthogonal frequency division multiple access (OFDMA) uplink in which multiplexing is performed by orthogonalizing a plurality of user signals (data) by the OFDM, if frequency synchronization and signal synchronization between respective user signals are insufficient, inter-carrier interference occurs.
As one means for solving the above problems, for example, there is a method of suppressing inter-carrier interference by using a null carrier (subcarrier not power-transmitted) (see Patent Document 1 mentioned below).
According to Patent Document 1, a transmitting station avoid occurrence of interference by inserting the null carrier into a known subcarrier portion where interference with a subcarrier transmitted from another transmitting station occurs, to perform data transmission.
Patent Document 1: Japanese Patent Application Laid-open No. H9-18433